The scheduler class defines a generic interface to scheduling events.
It needs two functions to actually deal with the “outside world” — timefunc
should be callable without arguments, and return a number (the “time”, in any
units whatsoever). If time.monotonic is not available, the timefunc default
is time.time instead. The delayfunc function should be callable with one
argument, compatible with the output of timefunc, and should delay that many
time units. delayfunc will also be called with the argument 0 after each
event is run to allow other threads an opportunity to run in multi-threaded
applications.

Changed in version 3.3: timefunc and delayfunc parameters are optional.

Changed in version 3.3: scheduler class can be safely used in multi-threaded
environments.

Schedule a new event. The time argument should be a numeric type compatible
with the return value of the timefunc function passed to the constructor.
Events scheduled for the same time will be executed in the order of their
priority.

Executing the event means executing action(*argument,**kwargs).
argument is a sequence holding the positional arguments for action.
kwargs is a dictionary holding the keyword arguments for action.

Return value is an event which may be used for later cancellation of the event
(see cancel()).

Run all scheduled events. This method will wait (using the delayfunc()
function passed to the constructor) for the next event, then execute it and so
on until there are no more scheduled events.

If blocking is false executes the scheduled events due to expire soonest
(if any) and then return the deadline of the next scheduled call in the
scheduler (if any).

Either action or delayfunc can raise an exception. In either case, the
scheduler will maintain a consistent state and propagate the exception. If an
exception is raised by action, the event will not be attempted in future calls
to run().

If a sequence of events takes longer to run than the time available before the
next event, the scheduler will simply fall behind. No events will be dropped;
the calling code is responsible for canceling events which are no longer
pertinent.